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Pregnancy and Breastfeeding

There are several stages in a woman’s reproductive life that influence her risk of developing breast cancer. These stages can increase or decrease that risk in different ways.

A woman’s life stages are related to hormonal changes that occur naturally, such as when you start and stop menstruating if or when you become pregnant and carry the pregnancy to full term, and if or when you are able to breastfeed.

There is evidence that pregnancy and childbirth influence your risk of breast cancer. Women who do not have children have a low increase in the risk of developing breast cancer compared to women who have had a child before the age of 35.

Pregnancy and reduced breast cancer risk

Research shows that the risk of breast cancer is reduced in women who have had children, particularly if they gave birth before the age of 30. Having more than one child also decreases your breast cancer risk and with every subsequent child your risk decreases further.

The hormones involved in a pregnancy carried to full-term mature the breast tissue in a way that seems to protect against breast cancer.  

Pregnancy and increased breast cancer risk

Like women who have no children, women who have their first child at the age of 35 or older have a low increase in the risk of developing breast cancer.

There is also a short-term increase in the risk of developing breast cancer after childbirth. This period of increased risk may last for up to several years following the birth of your child, and is likely a result of hormonal changes that promote cell growth in breast tissue.

Breastfeeding offers protective benefits for the mother

If breastfeeding is an option for you and your baby, evidence suggests that it can reduce your risk of developing breast cancer. 

The biggest benefits are from longer periods of breastfeeding, for a year or more with one child or over several births. The effect of breastfeeding on breast cancer risk may be related to the fact that women usually stop menstruating while they are still breastfeeding: this lowers a woman’s total number of menstrual periods over her life. Fewer menstrual periods means less exposure to estrogen. Breast tissue is susceptible to the effects of estrogen and estrogen exposure plays a role in the development of breast cancer.

The Canadian Breast Cancer Foundation encourages you to learn about your breast health, breast cancer risk, ways to reduce your risk, and the benefits and limitations of screening for the earlier detection of breast cancer. To inform your decisions, we also encourage you to consider speaking to a health care provider.

Established risk factors 

Non-modifiable Risk Factors Modifiable Risk Factors

Gender and age

Body weight

Personal cancer history

Physical activity

Family cancer history and genetics

Alcohol use

Early menstruation and late menopause

Smoking

Breast density

Exposure to hormones: the Pill, IVF, and HRT

Breast conditions

Pregnancy and breastfeeding

 

Radiation exposure

Sources

Canadian Breast Cancer Foundation – Ontario Region. (2010). Earlier Detection and Diagnosis of Breast Cancer: A Report from It’s About Time! A Consensus Conference. 

National Cancer Institute (US). Pregnancy and Breast Cancer Risk.  Accessed July 31, 2011.

Collaborative Group on Hormonal Factors in Breast Cancer. (2002). Breast cancer and breastfeeding. In The Lancet, Vol. 360, No. 9328. Accessed July 31, 2011.